Survivor Coop Bug

Recently, I have finished Survivor Coop with 3 other people. At round 480, the game froze and we had to leave the match.

Here's some proof.
http://imageshack.us/photo/my-images/28/buglo.jpg/

Is it intentional to freeze the game or...

+1

Same problem on Survivor Coop for the map BIOHAZARD, the game just froze at round 480 :/

..

Also there were no exp points added.